Various sensors and gauges use delay measurements of acoustic and optic pulses or coded signals. In some applications, the measurement of a distance is coupled with pattern detection. This is the case, for example, in some time-of-flight techniques that use visible or infrared light. Due to the high velocity of light, the read-out circuitry has to work in a time-critical domain. The fast capture and evaluation of photo-generated charge carriers is a particular focus of cell design and read-out technique. Background current from carriers outside the space charge regions is to be avoided because carrier diffusion is time-consuming.
This is a challenging task, in particular, for infrared light because of its penetration depth, on the order of ten microns or more. Spreading an electric field from the place where photo-generated charge carriers have to be gathered to the penetration depth of infrared light, or deeper, is a challenging task. It is desired to address this disadvantageous situation, given that infrared light is the signal of choice in many applications because of its invisibility.
Conventional solutions use transient switching modes: at fast bias sweep conditions the semiconductor region underneath a metal-insulator-semiconductor (MIS) electrode is pulsed into a deep depletion state. In this operation mode the depletion width is larger than the maximum depletion width under equilibrium. This effect is used at devices with surface electrodes for carrier capturing, i.e., charger coupled devices (CCDs) or a photonic mixer device. As can be seen in FIG. 1A, infrared light generates electron hole pairs at least partly outside the space charge region at moderate substrate doping levels. These carriers contribute to noise and should be avoided. Additionally, and referring to FIG. 1B, three-dimensional formation of space charge regions can also occur, implying cross-talk in some device geometries and limiting the shrinking potential of the device. Therefore, there is a need for improved photo cell devices.